I wonder why this was never replaced.

my Guess is it's MV...though i know they made HPS Silverliners in the 1970's iv'e seen very few HPS Silverliners....as for why it wasn't replaced,im sure there are various reason's why an old fixture isn't swapped with a new one,if you look around most cities you will find at least a few old lights hanging around,in less developed or older areas even more so,and it is also pretty expensive to upgrade lighting infrastructure as with anything else so they might go by the "it works so it stays" Policy to save money,other times in larger cities i see tacky methods of keeping old lights running like Wire strung between poles where the underground wiring has failed,doors held shut with tape and whatnot,in cities like dallas this is very apparent,as well as shreveport.

I know some brand new OVH's with electric take holding them shut! In Lakewood on Colefax. I dunno why the hinges failed already.

It could be HPS if it was never replaced, we have some HPS Westinghouse Silverliners here, in fact I own one

I've seen HPS versions of these before.

At which dates did they make these OV-25's in HPS? This is an HPS OV-25, and Ft Collins CO is COVERED in HPS OV-25's.

When did High Pressure Sodium come out? What year?